1. Field of the Invention
The present invention relates to pipes, particularly well suited for application in sanitary plumbing, made of phosphorus copper or phosphorus copper alloys and having good corrosion resistance properties. This invention also relates to a process of making such pipe.
2. Description of the Prior Art
Phosphorus copper or phosphorus copper alloy pipes are widely used in plumbing, particularly for hot and cold water pipes. Generally they are available in two forms, either as coils of annealed copper pipe or in a straight length. The pipes may be bare or provided with an insulating sheath of PVC, notably when they are designed to be embedded in concrete. Mostly they are made with a "clean" interior surface, that is to say, free from deposits.
It is well known in the art that in the course of fabricating these pipes, which includes a drawing operation between a die and a mandrel forming an annular space therebetween, the pipes come into contact with the drawing oil or lubricant. This lubricant is generally comprised of hydrocarbons, and is necessary for satisfactory operation of the drawing process. When such pipes are subsequently used in contact with chemically agressive liquids the appearance of pitting or of blisters is generally observed. Research has revealed that the carbon content of the interior surface of such pipes, which results from the distillation and decomposition of the drawing oils, particularly during the annealing process, generally increases the corrosion tendencies of the pipes. It is generally accepted that it is undesirable for the carbon content to exceed 2 mg/dm.sup.2.
The prior art discloses various methods of eliminating the drawing oil residues and especially avoiding the formation of carbon deposits derived from these oils.
One such method involves injecting into the pipes, prior to annealing, a degreasing agent such as trichloroethylene. This reduces residual oil which give rise to carbon deposits. After the degreasing fluid is passed through the pipe a current of steam is blown through the pipe to sweep out any residual traces of the solvent.
Another method involves injecting at high velocity a mixture of air and abrasive particles, such as alumina or silicon carbide, after the annealing process. This removes carbon deposits by abrasion. However, this method is impractical for use with coiled pipes and is solely limited to use with straight pipe lengths.
Finally, by injecting a gaseous mixture of oxygen and inert gases into the pipe prior to the heating treatment, it is possible to achieve residual amounts of carbon in the order of 0.3 mg/dm.sup.2. During this heat treatment step in a furnace, generally in a reducing atmosphere, the oxygen reacts with the residual hydrocarbons of the drawing lubricant which have remained inside the pipe and form volatile gases, such as carbon monoxide or carbon dioxide. These volatile gases are easily driven out of the pipe. It is critical to regulate the oxygen content in the gaseous mixture to prevent the formation of powderous oxide deposits inside the pipe during the heat treatment step. It is known that powderous oxide deposits, particularly in a thick layer are generally porous and have little adherence to the metal substrate. Such an oxide layer behaves similar to carbon deposits and gives rise to pitting corrosion. Research has shown that phosphorous copper pipes containing even less than 1 mg/dm.sup.2 carbon, but covered on their inside surface by powderous oxides formed during heat treatment, in slightly oxidizing conditions, have a tendency towards corrosion by pitting which is notably due to the presence of such oxide film.
The major disadvantage of the above prior art methods is that the carbon content is not sufficiently reduced which results in some degree of corrosion. Furthermore, the present state of the art has shown a desire to avoid the formation of an internal oxide layer.